With the recent spread of portable electronic equipment such as notebook computers, video camcorders, and personal digital assistances, nonaqueous electrolyte secondary batteries having high voltage and high energy density have come to be used widely as a power source. From the concern for the environmental protection, electric-powered vehicles and hybrid-powered vehicles utilizing electric power as a part of motive power have already been put to practical use.
Various additives for electrolyte have been proposed to provide nonaqueous secondary batteries with improved stability or electrical characteristics. Examples of such additives include 1,3-propane sultone (see patent document 1 below), vinyl ethylene carbonate (see patent document 2 below), vinylene carbonate (see patent document 3 below), 1,3-propane sultone or butane sultone (see patent document 4 below), vinylene carbonate (see patent document 5 below), and vinyl ethylene carbonate (see patent document 6 below). Among them, vinylene carbonate is widely used for its great effects. These additives are considered to form a stable film called a solid electrolyte interface covering the surface of a negative electrode, which film is expected to prevent reductive decomposition of an electrolyte.
A nonaqueous secondary battery can be charged to too high a voltage when excessive current is supplied due to, for example, incorrect operation. Such a phenomenon is called overcharge. Because an overcharged condition would greatly reduce the safety of a nonaqueous secondary battery, a nonaqueous secondary battery is provided with a mechanism for interrupting charging current when the voltage exceeds a preset threshold.